The present invention pertains to drilling operations and, more particularly, to the synchronization of clocks over a downhole network. In modern drilling, much importance is placed on the gathering of accurate, concise, and detailed information about the drilling environment. This information may prove itself to be invaluable upon making decisions relating to drilling operations and strategy. For example, detailed information regarding the shape and content of a natural formation can be a valuable resource for electing a point of entry into the formation.
Such information is often collected using seismic measurements, a practice well known in the art. Typically, a seismic source is used to send acoustic waves through an earth formation. The waves disperse through the formation, bouncing off of various features of the formation and causing reflection waves which are detected by sensors situated at different locations relative to the formation. The data from the reflection waves is analyzed to determine physical characteristics of the formation.
Such measurements are most advantageous when they are compared with other measurements from different locations during the same time interval. In order for seismic and other measurements to provide an accurate depiction of the object of interest, precise time-specific data are ideal. Often this involves the use of synchronized clocks in which the data taken from several locations is overlaid for a given time interval. However, some of the most useful seismic measurements in a drilling environment are recorded downhole. A typical drill string can be as long as 10,000 to 20,000 feet. Due to extremes in temperature, pressure, vibration, jolts, and other factors, a downhole clock can very easily develop a drift from a clock located on the surface or at another location on the drill string. Various methods of establishing and maintaining a system of two or more synchronized clocks in a drilling environment are recognized in the art.
U.S. Pat. No. 6,614,718 to Cecconi et al. discloses a device and method allowing seismic measurements to be performed in a well during drilling, comprising, above ground, a seismic source and a recorder connected to one or several seismic reference sensors and, downhole, a seismic sensor mounted in a drill string, wherein the seismic sensors and recorders comprise a low drift synchronized clock system, characterized in that the clock comprising the underground seismic sensor is a dual mode type clock.
U.S. Pat. No. 6,400,646 to Shah et al. discloses an acoustic system for synchronizing a clock in a well containing a drill string with a clock located near the surface of the well. The system includes devices for transmitting and receiving a pair of acoustic signals between locations comprising each clock and processing those signals.
U.S. Pat. No. 6,424,595 to Chenin discloses a method for downhole clock synchronization without the use of an electrical connection. The clocks are synchronized using acoustic and mud pulse communication methods.
U.S. Pat. No. 6,439,046 to Kruspe, et al. discloses an apparatus and method for synchronized formation measurement wherein a sensor is mounted on a drill string for sensing a parameter of interest. Two clocked controllers are synchronized such that the surface controller is performing certain tasks in timed sequence with the sensor even though the surface controller is not connected to the sensor.
Further, several types of systems for taking downhole measurements currently exists. Often logging instruments are lowered into a well bore during a pause in drilling. Another system is described in U.S. Pat. No. 6,670,880 to Hall, et al., which discloses a downhole data transmission system in which data can be transmitted through a string of downhole components. In one embodiment of the ″880 patent, a tool string comprises a plurality of downhole components, each with a first transmission element in one end and a second transmission element in another end. Each transmission element comprises a magnetically conductive, electrically insulating trough that houses an electrically conductive coil. The coils in each end of the component are connected by an electrical conductor. When the components are connected in a drill string, data transmission is enabled through the drill string. When measurements are made downhole, they may be conveyed to the surface by means of the system incorporated into the tool string.